Methods and apparatus for detecting ventricular depolarizations during atrial pacing

ABSTRACT

AV synchronous, dual chamber pacing systems are disclosed having improved sensing of ectopic ventricular depolarizations or PVCs coincidentally occurring at or shortly following delivery of an A-PACE pulse. A first ventricular sense amplifier that is blanked during and following delivery of an A-PACE pulse is coupled to active and indifferent ventricular pace/sense electrodes defining a ventricular sense vector for sensing natural ventricular depolarizations and declaring a V-EVENT. A far field PVC sense amplifier coupled to a far field PVC sense electrode pair defining a PVC sense vector detects such PVCs while the ventricular sense amplifier is blanked. A PVC declared during the ventricular blanking period by the far field PVC sense amplifier is employed to deliver a VSP pulse upon time-out of a VSP delay, if the VSP function is provided and programmed ON, and/or to halt time-out of an AV delay.

RELATED APPLICATIONS

This disclosure is related to the following co-pending U.S. patentapplication Ser. No. 11/260,984, filed Sep. 30, 2002, entitled “METHODAND APPARATUS FOR PERFORMING STIMULATION THRESHOLD SEARCHES” by C. M.Manrodt et al., which is not admitted as prior art with respect to thepresent disclosure by its mention in this section.

FIELD OF THE INVENTION

This invention relates to implantable AV synchronous, dual chamberpacing systems, and particularly to improved sensing of ectopicventricular depolarizations coincidentally occurring at or shortlyfollowing delivery of an atrial pacing pulse.

BACKGROUND OF THE INVENTION

Atrial synchronized, dual chamber, pacing modes, particularly, themulti-programmable, VDD, VDDR, DDD and DDDR pacing modes, have beenwidely adopted in implantable dual chamber pacemakers for providingatrial and ventricular or AV synchronized pacing on demand. Such dualchamber pacing modes have also been incorporated into implantablecardioverter/defibrillators (ICDs) and into right and left heart pacingsystems providing synchronized right and left heart pacing for enhancingleft ventricular cardiac output as described in commonly assigned U.S.Pat. No. 5,902,324.

Such pacing systems are embodied in an implantable pulse generator (IPG)adapted to be subcutaneously implanted and at least atrial andventricular pacing or cardioversion/defibrillation leads that arecoupled to the IPG. The atrial and ventricular leads each incorporateone or more lead conductor that extends through the lead body to anexposed pace/sense electrode or cardioversion/defibrillation electrodedisposed in operative relation to a heart chamber. Typically, anegative-going or cathodal voltage pacing pulse is applied through apacing path comprising a small surface area, active pace/sense electrode(also characterized as a cathode electrode) and a relatively largersurface area, return or indifferent pace/sense electrode (alsocharacterized as an anode electrode) to pace a heart chamber.

Such leads are typically characterized as unipolar leads if theycomprise only a single active pace/sense electrode and/or acardioversion/defibrillation electrode. In the pacing context, aunipolar lead is coupled with a unipolar IPG, wherein the electricallyconductive IPG housing or “can” comprises a return or indifferentpace/sense electrode or anode electrode. Unipolar pacing and sensingtakes place between the lead-borne active pace/sense electrode and thehousing indifferent pace/sense electrode. A bipolar lead comprises atleast two lead conductors coupled to a bipolar IPG and extending to anactive pace/sense electrode, typically located at the distal end of thelead body, and an indifferent pace/sense electrode, typically located onthe lead body proximal to the distal active pace/sense electrode.Bipolar pacing and sensing takes place between the lead-borne activepace/sense electrode and indifferent pace/sense electrode. In thebipolar configuration, the indifferent pace/sense electrode is usually aring-like structure, referred to as the “ring” electrode, locatedproximal to the distal active pace/sense electrode, by about 0.5 cm to2.5 cm. In this context, bipolar and unipolar sensing may also bereferred to as “near-field” and “far-field” sensing, respectively.(Although “far-field” usually denotes sensing outside the chamber ofinterest, and the unipolar signal derived from such a unipolarpace/sense electrode pair is dominated by the near-field tip electrodesignal.)

A pacing IPG capable of pacing in atrial synchronized modes typicallyincludes atrial and ventricular sense amplifiers, atrial and ventricularpace pulse generators or “amplifiers”, an operating system governingpacing and sensing functions, and components as described further hereinin relation to a preferred embodiment of the invention.

In the typical dual chamber DDD pacing system, an atrial pacing (A-PACE)pulse generated by the atrial pace pulse generator is applied to theright atrial active and indifferent pace/sense electrodes to cause theright and left atria to depolarize. Similarly, a ventricular pacing(V-PACE) pulse generated by the ventricular pulse generator is appliedto the right ventricular active and indifferent pace/sense electrodes tocause the right and left ventricles to depolarize. In more recentlydeveloped right and left heart pacing systems, pacing pulse generatorsand leads are incorporated into the pacing system to provide A-PACEand/or V-PACE pulses to the left atrium and/or ventricle.

The atrial sense amplifier is coupled to atrial active and indifferentpace/sense electrodes to detect electrical signals of the heartassociated with atrial depolarizations (P-waves) and to generate anatrial sense event (A-EVENT) signal when detection criteria are met. Theventricular sense amplifier is coupled to ventricular active andindifferent pace/sense electrodes to detect electrical signals of theheart associated with ventricular depolarizations (R-waves) and togenerate a ventricular sense event (V-EVENT) signal when detectioncriteria are met.

The pacing operating system times out various intervals from eachA-EVENT, V-EVENT, A-PACE, and V-PACE to maintain synchronousdepolarizations of the atria and ventricles. Such AV synchronouspacemakers that perform this function have the capability of trackingthe patient's natural sinus rhythm and preserving the hemodynamiccontribution of the atrial contraction over a wide range of heart rates.Maintenance of AV mechanical synchrony is of great importance as setforth in greater detail in commonly assigned U.S. Pat. No. 5,626,623.

Typically, the IPG operating system comprises a microcomputercontrolled, digital controller/timer circuit that defines and times outa V-A interval (in DDD and DDDR modes) or a V-V interval (in VDD andVDDR modes) upon a V-EVENT or V-PACE pulse and times out an AV delay inresponse to an A-EVENT (in VDD, VDDR, DDD, DDDR modes) or in response toan A-PACE pulse (in DDD and DDDR modes) as well as a number of otherintervals. An SAV delay is commenced by declaration of an A-EVENT, and aPAV delay is commenced upon delivery of the A-PACE pulse in certain DDDand DDDR pacing systems.

The A-PACE and V-PACE pulses are produced by the exponential dischargeof respective atrial and ventricular output capacitors through theimpedance loads in the atrial and ventricular pacing paths that eachinclude a coupling capacitor, the active and indifferent pace/senseelectrodes, and the patient's heart tissue between the pace/senseelectrodes. In conventional dual chamber pacing systems, both the atrialand ventricular sense amplifiers are “blanked”, i.e., uncoupled, fromthe respective atrial and ventricular pace/sense electrode pairs duringthe delivery of either of an A-PACE pulse or a V-PACE pulse and for aprogrammed blanking period thereafter. The gains of the atrial andventricular sense amplifiers are normally tuned for the relatively lowvoltages of the heart (e.g., 0.3 mV-4.0 mV for the atrial senseamplifier and 1.0 mV-20.0 mV for the ventricular sense amplifier). Thesignificantly greater voltages of the A-PACE and V-PACE pulses (e.g.,varying between 0.5 V and 8.0 V) must be blocked from the atrial andventricular sense amplifiers.

Moreover, a residual post-pace polarization signal (or“after-potential”) remains in the pacing path due to the residual energyin the impedance load that the output capacitor is discharged into todeliver the A-PACE or V-PACE pulse. The impedance load across the outputamplifier terminals comprises the impedance of the coupling capacitor,the lead conductor(s), the tissue-electrode interface impedances, andthe impedance of the body tissue bulk between the active and indifferentpace/sense electrodes. The impedances of the body tissue and the leadconductor(s) may be modeled as a simple series bulk resistance, leavingthe tissue-electrode interfaces and any coupling capacitors as thereactive energy absorbing/discharging elements of the total load. Thereare typically two tissue-electrode interfaces in a pacing path, one atthe active tip electrode, and one at the indifferent ring (or IPG caseor “can”) electrode. The energy stored in these interfaces and anycoupling capacitors dissipates after the pacing pulse through the pacingpath impedance load creating the after-potential that can be sensed ateach electrode and affect the ability of the sense amplifiers to sensenatural or evoked cardiac events. The tip electrode is the primaryafter-potential storage element in comparison to the case and ringelectrodes. An indifferent ring electrode typically stores more energythan does a can electrode due to differences in electrode areas.

Most current pacemaker output circuits incorporate “fast recharge”circuitry for short-circuiting the pacing path and actively dissipatingor countering after-potentials during the blanking of the senseamplifier's input terminals to shorten the time that it would otherwisetake to dissipate after-potentials. The primary purposes of providing arecharge operation are to ensure that the coupling capacitor(s) isrecharged to an insignificant voltage level or equilibrium prior to thedelivery of the next pacing pulse through it and to allow the net DCcurrent in the pacing path to settle to zero to facilitate sensing inthe same pacing path or using one of the pace/sense electrodes of thatpacing path.

Thus, it is conventional to suppress or blank both of the atrial andventricular sense amplifiers during A-PACE and V-PACE pulses forblanking periods to avoid overloading the sense amplifier. Moreover, thesense amplifiers may abruptly sense a different potential than waspresent at the time of initial blanking when the blanking period expiresand the sense amplifier is reconnected due to the after-potentials andelectrode polarization as well as the recharge function. This canproduce unwanted oversensing of artifacts resulting in falsedeclarations of A-EVENTs or V-EVENTs. Therefore, the blanking periods inpacemaker IPGs sold by the assignee of this application are nominallyset at 30 ms after delivery of an A-PACE or V-PACE, but the blankingperiods may be programmed as long as 45 ms in difficult sensingscenarios. There may be additional digital blanking of the senseamplifiers to avoid sensing of evoked response or other pacingartifacts, e.g., for 150 ms to 400 ms after paced events in ICDs. Suchblanking periods are characterized as an atrial blanking periods (ABP)including a post atrial pace, atrial blanking period (PAABP or PAAB) anda post ventricular pace, atrial blanking period (PVABP or PVAB) or as aventricular blanking periods (VBP) including a post atrial pace,ventricular blanking period (PAVBP or PAVB), and a post ventricularpace, ventricular blanking period (PVVBP or PVB).

In addition, a number of sense amplifier refractory periods are timedout on atrial and ventricular sense event signals and generation ofA-PACE and V-PACE pulses, whereby “refractory” A-EVENT and V-EVENTsduring such refractory periods are selectively ignored or employed in avariety of ways to reset or extend time periods being timed out. Atrialand ventricular refractory periods (ARP and VRP) are commenced upon anA-EVENT or V-EVENT or generation of an A-PACE or V-PACE pulse,respectively. The ARP is typically only employed by itself during atrialdemand pacing in the AAI pacing mode. In dual chamber pacing modes, theARP commenced by the A-EVENT or A-PACE pulse extends through the SAVdelay or the PAV delay until a certain time following a V-EVENTterminating the SAV or PAV delay or generation of a V-PACE pulse at theexpiration of the SAV or PAV delay. This post-ventricular atrialrefractory period (PVARP) is commenced by a V-PACE pulse or V-EVENTbased on the understanding that A-EVENTs sensed during its time-outgenerally reflect a retrograde conduction of the evoked or spontaneousventricular depolarization wave and therefore are not employed to resetan escape interval and commence an SAV delay. The duration of PVARP maybe fixed or vary as a function of sensed atrial rate or pacemakerdefined pacing rate, with the result that in many cases relatively longPVARPs are in effect at lower rates. A total ARP (TARP) is defined asthe entire duration of the ARP and the PVARP. See, for example, U.S.Pat. No. 6,311,088. Typically the ARP and VRP are set at 300 ms, and thePVARP durations are programmable in the range of 250 ms-400 ms.

The rate-adaptive VDDR, DDIR, and DDDR pacing modes function in theabove-described manner but additionally provide rate modulation of apacing escape interval between a programmable lower rate and an upperrate limit (URL) as a function of a physiologic signal or rate controlparameter (RCP) related to the need for cardiac output developed by aphysiologic sensor. At times when the intrinsic atrial rate isinappropriately high or low, a variety of “mode switching” schemes foreffecting switching between tracking modes and non-tracking modes (and avariety of transitional modes) based on the relationship between theatrial rate and the sensor derived pacing rate have been proposed asexemplified by commonly assigned U.S. Pat. No. 5,144,949.

In order to maximize the useful life of pacing IPGs, it is desirablethat the A-PACE and V-PACE pulse energies be programmed to the minimalenergies required to evoke a depolarization of the atria and ventricles(i.e., to “capture” the atria and ventricles). The minimum output pulseenergy which is required to capture and thus evoke a musculardepolarization within the heart is referred to as the stimulationthreshold, and generally varies in accordance with the well knownstrength-duration curves, wherein the amplitude of a stimulationthreshold current pulse and its duration are inversely proportional. Onedifficulty that arises from use of the blanking and refractory periodsrelates to the inability to use the sense amplifiers to detect thecapture or loss of capture (LOC) of the atria and ventricles.

Therefore, it has been proposed to employ additional sense electrodesand sense amplifiers or differing combinations of pace/sense electrodesor cardioversion/defibrillation electrodes to sense the evoked responseto a V-PACE or A-PACE as described in commonly assigned U.S. Pat. Nos.5,331,966 and 5,683,431. A subcutaneous electrode array (SEA) formed onthe surface of the IPG housing is proposed in the '966 patent forsensing the “far field” EGM at a distance from the heart along vectorsselected from the electrodes of the SEA. The far field EGM is employedfor a variety of reasons as set forth in the above-referenced '966patent. The '966 patent also describes a number of other sensing schemesin the prior art for sensing the electrical activity of the heart fordetermining LOC.

U.S. Pat. No. 3,949,758 relates to a threshold-seeking pacemaker withautomatically adjusted energy levels for pacing pulses in response todetected LOC, and describes separate sensing and pacing electrodes,which are each utilized in unipolar fashion with a third commonelectrode having a comparatively larger dimension, to reduce residualpolarization problems.

U.S. Pat. No. 3,977,411 discloses a pacemaker having separate sensingand pacing electrodes that are each utilized in unipolar fashion. Thesensing electrode comprises a ring electrode having a relatively largesurface area (i.e., between 75 to 200 mm²) for improved sensing ofcardiac activity (R-waves), and is spaced along the pacing leadapproximately 5 to 50 mm from the distally-located tip electrode usedfor pacing.

U.S. Pat. No. 3,920,024 discloses a pacemaker having a thresholdtracking capability that dynamically measures the stimulation thresholdby monitoring the presence or absence of an evoked response (R-wave).Various electrode configurations are illustrated in FIGS. 1B and 9A-9Ffor purposes of sensing the evoked response, including sensing isbetween an intracardiac electrode and a reference electrode that isspaced some distance away from the heart or sensing between intracardiacelectrodes.

U.S. Pat. No. 4,305,396 also relates to a rate-adaptive pacemakerwherein the output energy is automatically varied in response to thedetection or non-detection of an evoked response (R-wave) and thedetected stimulation threshold. It is stated to be preferred to use thesame electrode for both pacing and sensing, such as a unipolar orbipolar system wherein there is at least one electrode located in theventricle, but suggests that other lead designs may be utilized suchthat the sensing and pacing electrode are separate.

U.S. Pat. No. 4,387,717 relates to a pacemaker having a separate (i.e.,non-pacing) electrode element, implanted near or in direct contact withthe cardiac tissue, and positioned relative to the pacing electrodes(i.e., unipolar pacing from “tip” to “can”) to provide improved P-waveand R-wave sensing with minimal interference from the pacing electrodes.The “can” functions as an indifferent electrode for sensing incombination with the separate electrode element. The separate sensingelectrode is spaced from the pacing electrodes to minimize crosscoupling and interference from the pacing stimulus and after-potentials.The separate sensing electrode comprises an extravascular metallic platehaving a comparatively large surface area in one embodiment. In anotherembodiment the separate sensing electrode comprises a cylindrical metalring mounted on the insulated pacing lead between the pacemaker and the“tip” electrode, and is described as being located along the lead topermit positioning the sensing electrode either within the heart,externally on the heart wall, or in some remote location in the vascularsystem away from the heart.

U.S. Pat. No. 4,585,004 relates to an implantable cardiac pacing andmonitoring system, wherein the pace/sense electrodes are electricallyseparate from an auxiliary sense electrode system. The auxiliary senseelectrode system comprises a transvenous data lead with ring electrodesfor sensing located in the right ventricle (approximately 1 cm from thepacing tip electrode for R-wave sensing) and in the right atrium(approximately 13 cm from the tip electrode to be in close proximitywith the S-A node), both ring electrodes being used in conjunction withthe pacemaker can in unipolar sensing fashion.

U.S. Pat. No. 4,686,988 relates to a dual chamber pacemaker havingatrial and ventricular endocardial leads with a separate proximal ringelectrode coupled to a P-wave or R-wave sensing EGM amplifier fordetecting the atrial or ventricular evoked response to atrial orventricular stimulation pulses generated and applied to other electrodeson the endocardial lead system. The auxiliary lead system thus resemblesthe '004 patent.

U.S. Pat. No. 4,549,548 discloses a programmable DDD pacing system inwhich the selection of pace/sense electrodes is changed during eachpacing cycle to optimize the choice of unipolar and bipolar atrial andventricular operations. U.S. Pat. Nos. 4,759,366 and 4,858,610 relate toevoked response detector circuits that also employ fast recharge in atleast one separate sensing electrode in either unipolar or bipolarelectrode configurations in either or both the atrium and ventricle. Thecardiac pacing systems function as unipolar and bipolar systems atdifferent steps in the operating cycle. In the '610 patent, a separateelectrode on the connector block of the IPG can is suggested for use asthe reference electrode anode rather than the metal case itself if thecase is employed as the reference electrode for the delivery of thestimulation pulse. In the '366 patent, the detected evoked response isused in an algorithm for adjusting the pacing rate.

U.S. Pat. Nos. 4,310,000, 4,729,376, and 4,674,508 also disclose the useof a separate passive sensing reference electrode mounted on the IPGconnector block or otherwise insulated from the pacemaker case in orderto provide a sensing reference electrode which is not part of thestimulation reference electrode and thus does not have residualafter-potentials at its surface following delivery of a stimulationpulse. The aforementioned '000 patent suggests various modifications tothe passive sensing reference electrode depicted in its drawings,including the incorporation of more than one passive sensing referenceelectrode provided on or adjacent to the IPG can, positioned as deemednecessary for best sensing, and connected to one or more senseamplifiers. No specific use of the additional passive sensing referenceelectrodes is suggested, although the single passive sensing referenceelectrode is suggested for use with a sense amplifier to detect bothcapture and spontaneous atrial or ventricular electrical events in adual chamber pacing system.

Moreover, it has been proposed in the prior art to automatically selectamong pacing and sensing electrode pairs during the cardiac cycle or inresponse to a determination that lead impedance is unacceptable (whichmay arise from a lead fracture or electrode dislodgement or the like).See, for example, U.S. Pat. Nos. 4,958,632, 5,003,975, and 5,755,742 andthe above-referenced '548 patent. According to the '548 patent, theselection of unipolar or bipolar mode of operation is based on adetermination for monitoring the amplitude of sensed heartbeat signalsto determine whether the sensing operation would be performed better inthe unipolar or the bipolar mode. This is directed to a determination ofheart performance vis-a-vis the leads involved so as to control theselection of unipolar or bipolar sensing.

Thus, considerable effort has been expended in providing systems andmethods for overcoming the limitations on sensing imposed by delivery ofa pacing pulse across a pair of pace/sense electrodes for a variety ofpurposes, including detection of LOC and determination of pacingthresholds, determination of lead impedance, and selection of theoptimal pacing and sensing electrode pairs. Despite these improvements,pacing systems still employ the above-described atrial and ventricularblanking functions.

Disruption of AV electrical and mechanical synchrony frequently arisesdue to the spontaneous depolarization of the ventricles triggered at anectopic site in one of the ventricles. Such a spontaneous ventriculardepolarization that is not associated with a prior atrial depolarizationis characterized as a premature ventricular contraction (PVC). Many ofthe problems resulting from the occurrence of a PVC in a patient with adual chamber pacemaker are described more fully in U.S. Pat. Nos.4,788,980 and 5,097,832.

PVCs that occur during the V-A interval following a prior detectedR-wave or delivery of a V-PACE pulse are usually sensed as V-EVENTs thatrestart the V-A interval. PVCs that occur during the time-out of the AVdelay and following time-out of the PAVBP are indistinguishable fromsinus ventricular depolarizations that are conducted from the AV nodethrough the Bundle of His. The resulting V-EVENT inhibits delivery ofthe V-PACE, and the V-A interval is commenced.

As noted above, after-potentials on the ventricular pace/senseelectrodes at time-out of the PAVBP can erroneously be detected andresult in declaration of a V-EVENT by the ventricular sense amplifier.The pacing system will not provide appropriate ventricular pacing to apatient's heart having AV block if electrical noise or other signals aremistakenly sensed by the ventricular sense amplifier as V-EVENTS duringtime-out of the AV delay. The questionable nature and consequences ofmistakenly detecting V-EVENTs has led to the adoption of the practice ofdelivering a ventricular safety pace (VSP) pulse at a fixed time,typically 110 ms, following delivery of an A-PACE. In other words, a VSPpulse is delivered to the ventricular pace/sense electrodes if a V-EVENTis declared between the time-out of the PAVBP and a 110 ms VSP windowfollowing delivery of an A-PACE pulse. This 110 ms VSP window is oftendenoted the cross talk window. The 110 ms VSP window length is shorterthan the normal AV conduction time in humans, so any V-EVENT declaredwithin the VSP window is unlikely to be due to true AV conduction. Thedelivered VSP pulse captures the ventricles if the V-EVENT was due tocross talk, that is, sensing of the residual A-PACE energyafterpotentials. The delivered VSP pulse will not capture the ventriclesif the V-EVENT reflects a PVC, because the ventricles will be refractoryat that time. Thus, faced with this uncertainty, a VSP pulse isdelivered at time-out of the VSP window or delay so as to ensure thatthe ventricles are truly contracting at a safe time after delivery ofthe A-PACE pulse. The VSP function is a programmable feature of priorart pacing systems that may be programmed off by the physician ifdesired. One form of VSP operation is set forth in U.S. Pat. No.4,825,870, for example.

However, it frequently happens that the depolarization wavefront of aPVC reaches the pace/sense electrodes during the PAVBP, and theventricular sense amplifier does not detect the R-wave. Theafter-potentials from the PVC wavefront may not be strong enough at theventricular pace/sense electrodes to trigger a V-EVENT at time-out ofthe ventricular blanking period. Thus, a V-PACE pulse may be deliveredat the time-out of the AV delay. The AV delay may be programmed to belong enough so that the V-PACE is delivered during the vulnerable periodof the ventricles. The vulnerable period occurs during the T-waverepolarization of the ventricle (approx. 250 ms-400 ms). During thevulnerable period, there is a dispersion of refractoriness where somecardiac cells are repolarized while others are still refractory.Additional stimulation during this time has a higher likelihood ofinitiating a tachyarrhythmia than during periods where the cardiac cellsare either completely refractory or completely repolarized.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, AV synchronous, dual chamberpacing systems or any atrial based pacing system requiring ventricularsensing are provided having improved sensing of normal ventriculardepolarizations or ectopic ventricular depolarizations coincidentallyoccurring at or shortly following delivery of an A-PACE pulse.Ventricular activations can occur coincident with an A-PACE pulse orotherwise within the PAVBP in a number of scenarios, such as ectopicventricular depolarizations, also referred to as premature ventricularcontractions (PVCs) and normal ventricular activations during atrialunder-sensing or intermittent loss of atrial capture. For convenienceand because the most common form of under-sensed ventricular activationis due to PVCs, any such ventricular depolarization occurring coincidentwith the delivery of an A-PACE pulse is characterized herein as a PVC.

The QRS complex of such a PVC that appears between tightly spaced, nearfield, ventricular pace/sense electrodes is relatively narrow andexhibits a pronounced R-wave peak that is excellent for ventricularsensing when the ventricular sense amplifier is not blanked.Accordingly, the ventricular sense amplifier is preferably coupled withbipolar pace/sense electrodes and advantageously provides robust sensingof PVCs or conducted R-waves when it is not blanked. However, the narrowQRS complex sensed across the closely spaced ventricular pace/senseelectrodes dissipates by the time that the PAVBP times-out as thedepolarization wave front propagates through the ventricles and past theventricular pace/sense electrodes. Therefore, the R-wave peak of a PVCoccurring within the PAVBP is not sensed by the ventricular senseamplifier when the PAVBP times-out. A need therefore remains for acapability of sensing such PVCs falling within the PAVBP

We have observed that the QRS complexes of such PVCs observed acrosswidely spaced sense electrodes are relatively wide and are lesssusceptible to under-sensing during the PAVBP. We have also observedthat sense electrodes that are spatially separated from the ventricularpace/sense electrodes add additional sensing capabilities because of thepropagation delay of the QRS wavefront between such remote electrodes.In accordance with the present invention, a PVC occurring coincidentwith or shortly following delivery of an A-PACE pulse that would fallwithin the PAVBP is sensed employing a PVC sense amplifier that iscoupled to such widely spaced sense electrodes that do not include bothof the ventricular pace/sense electrodes coupled to the ventricularsense amplifier subjected to the PAVBP. The PVC sense amplifier may beblanked simply during delivery of the A-PACE pulse to protect the senseamplifier circuitry from the applied pacing voltage, but can then sensethe relatively wide QRS complex of the PVC that persists longer than theA-PACE pulse.

Therefore, in one embodiment of the present invention, a firstventricular sense amplifier is coupled to active and indifferentventricular pace/sense electrodes for sensing natural ventriculardepolarizations and declaring a V-EVENT. The first ventricular senseamplifier is blanked during the PAVBP following delivery of an A-PACEpulse. A far field or unipolar PVC sense amplifier coupled to a farfield, PVC sense electrode pair detects such PVCs while the ventricularsense amplifier coupled to the active and indifferent ventricularpace/sense electrodes is blanked. The far field PVC sense electrode pairis disposed in the patient's body to define a far field PVC sense vectordiffering from a ventricular sense vector defined by the active andindifferent ventricular pace/sense electrodes.

In another aspect of the present invention, the VSP function isadvantageously augmented by the redundant sensing capability provided bythe first ventricular sense amplifier and the PVC sense amplifier. Asdescribed above, when a PVC is under-sensed in a dual chamber pacingsystem, a V-PACE pulse is delivered at the end of the AV interval. Atnominal AV intervals, the ventricle is typically refractory to asubsequent V-PACE pulse. However, a V-PACE pulse delivered after a longAV interval has a greater probability of capturing the heart. The V-PACEpulse may be delivered within a patient's vulnerable period and incertain circumstances may initiate an arrhythmia in a susceptiblepatient. The mounting evidence suggesting long-term deleterious effectsof right ventricular apical pacing may increase physician motivation toextend the AV interval to decrease ventricular pacing. Ventricularsafety pacing ensures a ventricular beat for each cardiac cycle andensures that the V-PACE pulse is not delivered in the ventricularvulnerable period. This is accomplished by delivering the VSP pulseshortly after the A-PACE pulse when a V-EVENT is detected closelyfollowing the delivery of the A-PACE pulse. The subsequent VSP pulsewill capture the heart if a V-EVENT was declared due to noise, but thesubsequent VSP pulse will not capture the heart if the V-EVENT was dueto sensing of a PVC. In accordance with this aspect of the presentinvention, a VSP pulse is delivered if either of the ventricular senseamplifier that is subjected to the PAVBP or the PVC sense amplifierdeclares a V-EVENT. In this way, the sensing of such PVCs occurringcoincident with the delivery of A-PACE pulses is improved and thepotential for ventricular pacing during the vulnerable period isminimized.

In the simplest atrial pacing systems, the PVC sense electrode pair cancomprise one of the ventricular pace/sense electrodes and an indifferentelectrode supported on or comprising the conductive IPG can defining aunipolar PVC sense vector. Or, the PVC sense electrode pair can comprisea selected pair of sense electrodes of an SEA supported by the IPGenclosure defining an optimal PVC sense vector. Or, in an ICD contextproviding atrial pacing, the PVC sense electrode pair can comprise afurther cardioversion/defibrillation electrode pair defining an optimalPVC sense vector or can comprise one of the furthercardioversion/defibrillation electrodes and the indifferent electrodesupported on or comprising the conductive IPG can defining a optimal PVCsense vector. Or, in a right and left heart pacing context providingatrial pacing, the PVC sense electrode pair can comprise right and leftheart chamber pace/sense electrodes defining an optimal PVC sense vectoror can comprise one of the left heart chamber pace/sense electrodes andthe indifferent electrode supported on or comprising the conductive IPGcan defining an optimal PVC sense vector.

Preferably, the far field sense electrode pair can be selected in a testroutine or work-up by the physician commenced by programming a PVC senseelectrode pair coupled with the PVC sense amplifier and entering a testroutine. The results of the test routines of available PVC senseelectrode pairs can be compared to identify the optimal PVC sensevector.

As noted above, the ability to detect a PVC during the PAVBP can beemployed advantageously to trigger VSP pacing or to inhibit ventricularpacing, which in either case avoids delivery of a V-PACE pulse at thetime-out of the PAV delay possibly into the vulnerable period of theheart cycle. The ability to detect a PVC at other times during the PAVor SAV delay or the V-A interval can advantageously be employed toconfirm declarations of V-EVENTs, leading to more robust V-EVENTsensing.

Advantageously, the PVC sense amplifier can be enabled during thecardiac cycle, to function as a conventional EGM sense amplifier so thatthe spontaneously occurring PQRST complexes can be recorded for realtime analysis or data storage as is well known in the art.

This summary of the invention has been presented here simply to pointout some of the ways that the invention overcomes difficulties presentedin the prior art and to distinguish the invention from the prior art andis not intended to operate in any manner as a limitation on theinterpretation of claims that are presented initially in the patentapplication and that are ultimately granted.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other advantages and features of the present invention will bemore readily understood from the following detailed description of thepreferred embodiments thereof, when considered in conjunction with thedrawings, in which like reference numerals indicate identical structuresthroughout the several views, and wherein:

FIG. 1 is a schematic illustration of a dual chamber pacemaker implantedin a patient's chest comprising an IPG and endocardial leadstransvenously introduced into the right atrium and right ventricle ofthe heart, wherein PVC sensing can be conducted during the PAVBP acrossselected far field sensing electrode pairs;

FIG. 2 is a block diagram of the pacing IPG of FIG. 1 in which thepresent invention may be practiced;

FIG. 3 is flow chart depicting the steps of a DDD pacing cycle;

FIG. 4 is a detailed flow chart depicting the steps of detecting andresponding to a PVC sensed during the time-out of the PAVBP;

FIG. 5 is a schematic illustration of a further embodiment of a dualchamber pacemaker implanted in a patient's chest comprising an IPGsupporting a SEA and endocardial leads transvenously introduced into theright atrium and right ventricle of the heart, wherein PVC sensing canbe conducted during the PAVBP across selected far field SEA senseelectrode pairs;

FIG. 6 is a block diagram of the pacing IPG of FIG. 5 in which thepresent invention may be practiced;

FIG. 7 is a schematic illustration of a further embodiment of a dualchamber, right and left heart pacemaker implanted in a patient's chestand endocardial leads transvenously introduced into the right atrium,right ventricle and coronary sinus of the heart, wherein PVC sensing canbe conducted during the PAVBP across selected right and left heart senseelectrode pairs;

FIG. 8 is a block diagram of the pacing IPG of FIG. 7 in which thepresent invention may be practiced;

FIG. 9 is a schematic illustration of a dual chamber pacing ICDimplanted in a patient's chest comprising an IPG and endocardial leadstransvenously introduced into the right atrium, right ventricle, andcoronary sinus of the heart supporting pace/sense and/orcardioversion/defibrillation electrodes, wherein PVC sensing can beconducted during the PAVBP across selected far field sensing electrodepairs; and

FIG. 10 is a block diagram of an ICD IPG in which the present inventionmay be practiced.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, references are made toillustrative embodiments of methods and apparatus for carrying out theinvention. It is understood that other embodiments can be utilizedwithout departing from the scope of the invention.

FIGS. 1 and 2 depict the external configuration and components of atypical implantable dual chamber pacemaker operating in a DDD, DDI,DDIR, or DDDR pacing mode or operating in an MI or MIR pacing mode toprovide atrial pacing in the absence of an adequate atrial heart rate aslong as ventricular sensing indicates normal AV conduction. Such a dualchamber IPG 100 and unipolar or bipolar atrial and ventricular leads 114and 116 (bipolar leads are depicted), in which the present invention maybe implemented is depicted in FIGS. 1 and 2. The dual chamber pacemakerIPG 100 senses and paces in the atrial and ventricular chambers, andpacing is either triggered and inhibited depending upon sensing ofintrinsic, non-refractory atrial and ventricular depolarizations duringthe sequentially timed V-A interval and AV delay, respectively, as iswell known in the art, in accordance with the steps set forth in theflow chart of FIG. 3. The present invention functions when the atria arepaced due to failure to detect atrial depolarizations or when the sensedatrial heart rate falls below a rate dictated by a RCP related to theneed for cardiac output developed by a physiologic sensor.

In addition, the present invention can be implemented in such a dualchamber pacing system that is incorporated into a dual chamber pacingICD or into a right and left heart pacing system by itself or that isincorporated into a multi-chamber pacing IPG. The following descriptionis thus intended to encompass all of the various types of dual chamberpacemaker systems in which the present invention can be implemented.

The IPG 100 is provided with a hermetically sealed enclosure or can 118,typically fabricated of bio-compatible metal such as titanium, enclosingthe dual chamber IPG circuit 300 depicted in FIG. 2. A connector blockassembly 112 is mounted to the top of the can 118 to receive electricalconnectors located on the proximal connector ends of the depictedbipolar atrial and ventricular pacing leads 114 and 116.

As described further below, an electrically exposed area of the can 118functions as an IND_CAN electrode 140 that is electrically connected toone input of a PVC sense amplifier to facilitate sensing of PVCs overthe heart cycle, particularly to facilitate sensing PVCs during thePAVBP following delivery of an A-PACE pulse.

The bipolar atrial pacing lead 116 extends between its proximalconnector coupled to IPG 100 and distal atrial pace/sense electrodes 120and 122 located in the right atrium 12 of heart 10 to enable sensing ofP-waves and delivery of atrial pacing pulses to the right atria. Atrialpacing pulses may be delivered between electrodes 120 and 122 in abipolar pacing mode or between electrode 122 and the IND_CAN electrode140 of the IPG 100 in a unipolar pacing mode. Sensing of P-waves by theatrial sense amplifier subject to atrial blanking may occur betweenelectrode 120 and electrode 122 in a bipolar sensing mode or betweeneither of electrode 120 and 122 and the IND_CAN electrode 140 of the IPG100 in a unipolar atrial sensing mode.

Similarly, the bipolar ventricular pacing lead 114 extends between itsproximal connector coupled to IPG 100 and distal ventricular pace/senseelectrodes 128 and 130 located in the right ventricle 16 of heart 10 toboth sense R-waves and to deliver ventricular pacing pulses to theventricles. Ventricular pacing pulses may be delivered betweenelectrodes 128 and 130 in a bipolar pacing mode or between electrode 130and the IND_CAN electrode 140 of the IPG 100 in a unipolar pacing mode.Sensing of R-waves by the ventricular sense amplifier subject toblanking occurs between electrodes 128 and 130 in a bipolar sensing modein this preferred embodiment.

The IPG circuit 300 within IPG 100 and the bipolar atrial andventricular leads 114 and 116 are depicted in FIG. 2 in relation toheart 10. The IPG circuit 300 is divided generally into a microcomputercircuit 302 and a pacing input/output circuit 320. The input/outputcircuit 320 includes the digital controller/timer circuit 330, theatrial and ventricular pacing pulse output circuit 340 and the atrialand ventricular sense amplifiers circuit 360, as well as a number ofother components and circuits described below. The digitalcontroller/timer circuit 330 provides control of timing and otherfunctions within the input/output circuit 320. Digital controller/timercircuit 330, operating under the general control of the microcomputercircuit 302, includes a set of timing and associated logic circuits, ofwhich certain ones pertinent to the present invention are depicted anddescribed further below.

Preferably, the IPG 100 or one of the leads 114 or 116 includes one ormore physiologic sensor that develops a physiologic signal that relatesto the need for cardiac output. The use of physiologic sensors toprovide variation of pacing rate in response to sensed physiologicparameters, such as physical activity, oxygen saturation, blood pressureand respiration, has become commonplace.

Commonly assigned U.S. Pat. Nos. 4,428,378 and 4,890,617 discloseactivity sensors that are employed to vary the pacing escape interval insingle and dual chamber pacemaker IPGs in response to sensed physicalactivity. Such an activity sensor 316 is coupled to the inside surfaceof the IPG hermetically sealed enclosure 118 and may take the form of apiezoelectric crystal transducer as is well known in the art. Theactivity sensor 316 generates an output signal in response to certainpatient activities, e.g. ambulating, that is processed and used as arate control parameter (RCP). If the IPG operating mode is programmed toa rate responsive mode, the patient's activity level developed in thepatient activity circuit (PAS) 322 is monitored, and a sensor derivedV-A, A-A or V-V escape interval is derived proportionally thereto. Atimed interrupt, e.g., every two seconds, may be provided in order toallow the microprocessor 304 to analyze the output of the activitycircuit PAS 322 and update the basic V-A (or A-A or V-V) escape intervalemployed to govern the pacing cycle and to adjust other time intervalsas described below.

The bipolar leads 114 and 116 are illustrated schematically with theirassociated pace/sense electrode sets 120, 122 and 128, 130,respectively, as coupled directly to the atrial and ventricular pacingpulse output circuit 340 and sense amplifiers circuit 360 of pacingcircuit 320. The atrial and ventricular pacing pulse output circuit 340and sense amplifiers circuit 360 contain pulse generators and senseamplifiers corresponding to any of those presently employed incommercially marketed cardiac pacemakers for atrial and ventricularpacing and sensing.

Sense amplifiers circuit 360 also comprises a PVC sense amplifiercoupled with the IND_CAN electrode 140 and one of the ventricularpace/sense electrodes 128 or 130 selected by a ventricular select(V-SELECT) signal so that PVCs can be sensed along a bipolar orfar-field sense vector. It will be understood that other senseelectrodes can be coupled to the PVC sense amplifier within the senseamplifiers circuit 360 and selected by an appropriate V-SELECT signalthrough programming commands in the course of a telemetry session.

Sensitivity settings of the atrial and ventricular sense amplifiers andthe PVC sense amplifier in sense amplifiers circuit 360 can beprogrammed by the physician to reliably sense true P-waves, R-waves andPVCs during a patient work-up at implantation or during a patientfollow-up telemetry session. Digital controller/timer circuit 330controls the sensitivity settings of the atrial and ventricular senseamplifiers in sense amplifiers circuit 360 by means of sensitivitycontrol 350.

The depicted counters and timers within digital controller/timer circuit330 include ABP and VBP timers 366, intrinsic interval timers 368 fortiming average intrinsic A-A and V-V intervals from A-EVENTs andV-EVENTs, escape interval timers 370 for timing A-A, V-A, and/or V-Vpacing escape intervals, an AV delay timer 372 for timing the SAV delayfrom a preceding A-EVENT or PAV delay from a preceding A-TRIG,refractory period timers 374 for timing ARP, PVARP and VRP times and aPVC flag register 376 that is set upon detection of a PVC. Digitalcontroller/timer circuit 330 starts and times out these intervals andtime periods that are calculated by microcomputer circuit 302 forcontrolling the above-described operations of the atrial and ventricularsense amplifiers in sense amplifiers circuit 360 and the atrial andventricular pace pulse generators in output amplifier circuit 340.

In order to trigger generation of a V-PACE pulse, digitalcontroller/timer circuit 330 generates a V-TRIG signal at the end of aPAV or SAV delay provided by AV delay timer 372. Similarly, in order totrigger an atrial pacing or A-PACE pulse, digital controller/timercircuit 330 generates an A-TRIG signal at the termination of the V-Ainterval timed out by escape interval timers 370.

The ABP and VBP timers 366 of digital controller/timer circuit 330 timeout the above-described PAVBP and PAABP during and following an A-PACEpulse and the PAVBP and PVVBP during and following a V-PACE pulse. Thus,an atrial blanking (A-BLANK) signal is applied to the atrial senseamplifier for the prevailing ABP, and a ventricular blanking (V-BLANK)signal is applied to the ventricular sense amplifier for the prevailingVBP. In the absence of an A-BLANK signal, atrial depolarizations orP-waves that are detected by the atrial sense amplifier result in anA-EVENT that is communicated to the digital controller/timer circuit330. Similarly, in the absence of a V-BLANK signal, ventriculardepolarizations or R-waves that are detected by the ventricular senseamplifier result in a V-EVENT that is communicated to the digitalcontroller/timer circuit 330. In accordance with the present invention,the PVC sense amplifier within sense amplifiers circuit 360 is onlyblanked during delivery of the A-PACE pulse to prevent the deliveredA-PACE pulse from either damaging the sense amplifier circuitry or beingincorrectly sensed as a PVC.

The refractory period timers 374 time the ARP from an A-TRIG pulse orA-EVENT during which a sensed A-EVENT is ignored for the purpose ofresetting the V-A interval. The ARP extends from the beginning of theSAV or PAV interval following either an A-EVENT or an A-TRIG and until apredetermined time following a V-EVENT or a V-TRIG. The refractoryperiod timers 374 also time the PVARP from a V-TRIG pulse or V-EVENTduring which a sensed A-EVENT is also ignored for the purpose ofresetting the V-A interval. The VRP is also timed out by the refractoryperiod timers 374 after a V-EVENT or V-TRIG signal so that a subsequent,closely following V-EVENT is ignored for the purpose of restarting theV-A interval and setting the PVC flag in register 366.

The base ARP, PVARP and VRP that prevails at the lower rate of 60-70bpm, for example, are either default or programmed parameter valuesstored in the microcomputer 302. These refractory period parametervalues can be fixed for the full operating range of pacing rates betweenthe programmed lower rate and the URL, which may be 120 bpm, forexample, or they can be programmed to follow the algorithm forautomatically shortening in duration as the paced or intrinsic heartrate increases to ensure that the long refractory periods during thediminishing escape intervals do not prevent delivery of ventricularpacing pulses synchronized to valid intrinsic P-waves.

The A-EVENT is characterized as a refractory A-EVENT if it occurs duringtime-out of an ARP or a PVARP or a non-refractory A-EVENT if it occursafter time-out of these atrial refractory periods. Similarly, a V-EVENTis characterized as a refractory V-EVENT if it occurs during time-out ofa VRP or a non-refractory V-EVENT if it occurs after time-out of theventricular refractory period. Refractory A-EVENTs and V-EVENTs aretypically ignored for purposes of resetting timed out AV delays and V-Aintervals, although diagnostic data may be accumulated related to theiroccurrences.

Microcomputer 302 contains a microprocessor 304 and associated systemclock 308 and on-processor RAM and ROM chips 310 and 312, respectively.In addition, microcomputer 302 includes a separate RAM/ROM chip 314 toprovide firmware and additional RAM memory capacity. Microprocessor 304is interrupt driven, operating in a reduced power consumption modenormally, and awakened in response to defined interrupt events, whichmay include the A-TRIG, V-TRIG, A-EVENT and V-EVENTs.

Microcomputer 302 controls the operational functions of digitalcontroller/timer 324, specifying which timing intervals are employed ina programmed pacing mode via data and control bus 306. The specificvalues of the intervals timed by the digital controller/timer circuit330 are controlled by the microcomputer circuit 302 by means of data andcontrol bus 306 from programmed-in parameter values. The microcomputer302 also calculates the RCP derived or intrinsic atrial rate derivedV-V, A-A or V-A interval, the variable AV delay, and the variable ARP,PVARP and VRP. Typically, the AV delay in modern VDD, VDDR, DDD and DDDRpacemakers is either fixed or varies with the prevailing intrinsicatrial rate, measured as an A-A interval, and/or varies as a function ofa physiologic sensor derived pacing rate.

Digital controller/timer circuit 330 also interfaces with other circuitsof the input output circuit 320 or other components of IPG circuit 300.Crystal oscillator circuit 338 provides the basic timing clock andbattery 318 provides power for the pacing circuit 320 and themicrocomputer circuit 302. Power-on-reset circuit 336 responds toinitial connection of the circuit to the battery 318 for defining aninitial operating condition and similarly, resets the operative state ofthe IPG circuit 300 in response to detection of a low battery condition.Reference mode circuit 326 generates stable voltage reference andcurrents for the analog circuits within the pacing circuit 320. ADC(analog to digital converter) and multiplexer circuit 328 digitizesanalog signals and voltage to provide real time telemetry of cardiacsignals from sense amplifiers 360, for uplink transmission via RFtransmitter and receiver circuit 332. Voltage reference and bias circuit326, ADC and multiplexer 328, power-on-reset circuit 336 and crystaloscillator circuit 338 may correspond to any of those presently used incurrent marketed implantable cardiac pacemakers.

Data transmission to and from an external programmer (not shown) duringa telemetry session is accomplished by means of the telemetry antenna334 and an associated RF transmitter and receiver 332, which serves bothto demodulate received downlink telemetry and to transmit uplinktelemetry. Uplink telemetry capabilities will typically include theability to transmit stored digital information, e.g. operating modes andparameters, EGM histograms, and other events, as well as real time EGMsof atrial and/or ventricular electrical activity and Marker Channelpulses indicating the occurrence of sensed and paced depolarizations inthe atrium and ventricle, as are well known in the pacing art.

Reed switch 317 when closed by application of a magnetic field may beemployed to enable programming of the pacemaker and also may be employedto convert the pacemaker temporarily to an asynchronous pacing mode suchas DOO or VOO. Operation in the asynchronous mode may continue as longas the magnetic field is present, may continue until overridden by theprogrammer or may continue for a pre-set time period.

The illustrated IPG circuit 300 of FIG. 2 is merely exemplary, andcorresponds to the general functional organization of mostmulti-programmable microprocessor controlled DDD and DDDR cardiacpacemaker IPGs presently commercially available. It is believed that thepresent invention can readily be practiced using the basic hardware andsoftware of existing microprocessor controlled, dual chamber pacingsystems that are incorporated into dual chamber pacemakers or into ICDsor into right and left heart pacing systems. The invention is preferablyimplemented into the exemplary pacing system by means of modificationsto the hardware incorporating the PVC sense amplifier to detect signalsacross a PVC sense vector during the PAVBP and at other times during thepacing cycle and to declare a PVC if the signal (regardless of its truesource) satisfies PVC detection criterion. In addition, software storedin the ROM 310 of the microcomputer circuit 302 responding to suchdetected PVCs is modified as described further below. However, theoperating functions of the present invention may also be usefullypracticed by means of a full custom integrated circuit, for example, acircuit taking the form of a state machine, in which a state counterserves to control an arithmetic logic unit to perform calculationsaccording to a prescribed sequence of counter controlled steps.

FIG. 3 is a functional flow chart of the overall pacing cycle timingoperation of the pacemaker IPG circuit 300 illustrated in FIG. 2 in theDDD or DDDR pacing modes. In the flow chart of FIG. 3, it is assumedthat the A-A or V-V escape interval, cardiac cycle timing of the IPGcircuit 300 ranges between a programmed lower rate and a programmed URLand is based on the definition of a V-A interval and an AV delay,specifically either the SAV or the PAV delay interval. The AV delay andV-A interval of any given pacing cycle may be determined as a functionof a sensor-derived V-A interval or an atrial rate based V-A intervaldetermined by the average measured intrinsic A-A atrial rate if it isstable and varies between the programmed lower rate and URL. In thisparticular embodiment, separate SAV and PAV delays are defined, althoughin practice they may have the same duration. The operations of the flowchart may also incorporate any of the mode switching and sinuspreference algorithms of the prior art described above to switch betweenthe use of the sensor or the atrial rate derived escape intervals.However the algorithm is specifically implemented, it is understood toincorporate the PVC response algorithm of the present invention asdescribed hereafter.

For convenience, the pacing cycle is assumed to begin at step S100starting from a non-refractory A-EVENT. Timing of the prevailing SAVdelay and ARP are commenced in step SI 00, and the system awaits eithertime out of the SAV delay in step S102 or a non-refractory V-EVENT instep S104. Neither of the atrial and ventricular sense amplifiers isblanked, and the PVC sense amplifier may also be enabled. A V-TRIG andthe associated A-BLANK and V-BLANK signals are generated at step S106 atthe end of the SAV delay if a non-refractory V-EVENT does not occur atstep S104 prior to SAV time-out in step S102.

The SAV delay is terminated without delivery of a V-PACE pulse if eitherof a PVC or a V-EVENT is declared or if both a PVC and a V-EVENT aredeclared in step S124, and the V-A interval is restarted in step S108.The redundant sensing of PVCs or other signals by the PVC senseamplifier and the near field R-wave sense amplifier during time-out ofthe SAV delay provides a robust sensing capability that increasesconfidence that unnecessary pacing of the ventricles is avoided.

The V-A interval time-out is commenced in step S108, and time-out of thepost ventricular time periods including the VRP, PVARP, PAVBP and PVVBPare commenced in step S110. The algorithm awaits expiration of the V-Ainterval at step S112, and it is possible that a refractory ornon-refractory A-EVENT or V-EVENT can occur during the V-A intervaltime-out.

If a non-refractory A-EVENT is sensed in step S120 during time-out theV-A interval, the V-A interval is terminated, the AV delay is set to theSAV delay in step S124, and the SAV delay and associated post atrialsense ARP is timed out in step S100. Optionally, the non-refractoryA-EVENT also causes the V-A interval to be measured by intrinsicinterval timer 368 and employed to derive or update the intrinsic atrialrate that is saved in RAM. The V-A interval, the SAV and PAV delays, thePVARP, and the pacing escape interval for the next cardiac cycle canthen be recalculated in dependence upon either the updated average A-Ainterval or upon the RCP in a manner well known in the art.

In the absence of detection of a preceding A-EVENT, if a non-refractoryV-EVENT is declared sensed by the near field or bipolar ventricularsense amplifier at step S122 during time out of the V-A interval, thenthe declared V-EVENT is characterized as a PVC in step S124. It shouldbe noted that such a declaration of a V-EVENT during the V-A intervalcan be confirmed by the declaration of a PVC by the PVC sense amplifier.Certain algorithms, e.g., those disclosed in the above-referenced '088patent, have been devised to deal with such PVCs occurring during theV-A interval that could be practiced along with but are not necessary tothe practice of the present invention.

An A-TRIG signal is generated in step S114 at the time-out of the V-Ainterval if the V-A interval times out without sensing any suchintervening non-refractory A-EVENT or V-EVENT. In this case, the nextsucceeding AV delay is defined to be equal to PAV at step S116, and thePAV is timed out in step S118 along with the associated VSP delay andthe ARP, ABP and PAVBP in accordance with the steps of FIG. 4. Theparticular algorithm of FIG. 4 assumes that a VSP function is providedand that the VSP delay is timed out in timers 366 whenever a V-PACE isdelivered, but the present invention can be practiced without the VSPfunction being present or programmed on in a particular case. Moreover,the algorithm of FIG. 4 assumes that the PVC sense amplifier is alwaysenabled, but the PVC sense amplifier could be blanked or disabled duringdelivery of the A-PACE pulse and V-PACE pulse.

The time-out of the PAV delay is monitored in step S128, and a V-PACEpulse is delivered in step S138 if the PAV delay does time-out withoutdeclaration of either of a PVC or a V-EVENT. In step S130, a PVC can bedeclared at any time during the PAV delay and a V-EVENT can be declaredfollowing the time-out of the PAVBP. If the VSP function is not presentor programmed ON as determined in step S132, then such a declared PVC orV-EVENT would simply cause the V-A interval to commence in step S108.

However, preferably the VSP function is employed as determined in stepS132, and a declared PVC or V-EVENT causes the V-A interval to commencein step S108 only if it is declared after time-out of the VSP delay. Ifa PVC or V-EVENT is declared in step S130 before time-out of the VSPdelay, then a V-PACE is delivered in step S140 at time-out of the VSPdelay.

To enable this function, a VSP flag is set in step S136 if a PVC orV-EVENT is declared in step S130 before time-out of the VSP delay asdetermined in step S134. The status of the VSP flag is checked in stepS138 when the VSP delay does time-out as determined in step S134. Sincethe VSP flag was set in this instance in step S136, then the V-PACEpulse is delivered at time-out of the VSP delay. In this way, a PVC thatwould otherwise not be detected during the PAVBP does trigger the VSPfunction to pace the ventricles within a safe time from the PVC and notwithin the vulnerable period of the heart.

If a PVC or V-EVENT is declared in step S130 after time-out of the VSPdelay, as determined in step S134, then a V-PACE is not delivered instep S140. The time-out of the PAV delay is terminated, and the V-Ainterval is started in step S108. The redundant sensing of PVCs or othersignals by the PVC sense amplifier and the near field R-wave senseamplifier in the time period between the end of the VSP delay and thetime-out of the PAV delay provides a robust sensing capability thatincreases confidence that unnecessary pacing of the ventricles isavoided.

The PVC sense amplifier of the depicted embodiment of FIGS. 1 and 2senses the far field R-wave to particularly detect PVCs across thesensing vector comprising the IND_CAN electrode 10 and one of the ringand tip ventricular pace/sense electrodes 128 and 130. It is expectedthat the PVC sense amplifier could be advantageously coupled to theIND_CAN electrode 10 and the ring pace/sense electrode 128 because itmay not be in the blood and not in contact with endocardial surfaceresulting in a wide QRS complex. It will be understood from thefollowing that other far field sensing vectors can be selected dependingon the available sensing electrodes of the pacing orcardioversion/defibrillation system. The PVC sense amplifier sensitivitycan be programmed in a telemetry session to sense intrinsic R-wavesappearing in a conventional ECG display. The PVC sense amplifier'suplink telemetered response (the presence or absence of a PVC outputsignal) can be observed simultaneously. The PVC sense amplifiersensitivity can be varied for each programmed PVC sense vector, and thePVC sense vector providing the best consistent detection of R-waves canbe determined. A “permanent” V-SELECT can then be programmed forcoupling the PVC sense amplifier inputs to receive the optimal pair ofsignals across the PVC sense electrodes during chronic implantation.

The present invention including the steps of FIGS. 3 and 4, can bepracticed in a dual chamber pacemaker of the type depicted in FIGS. 5and 6 comprising an IPG 100′ supporting a SEA on the IPG housingcomprising at least one pair of sense electrodes whereby a sense vectoror sense vectors can be defined between the sense electrodes. The IPG100′ and IPG circuit 300′ conform in most ways to the IPG 100 and IPGcircuit 300 described above in reference to FIGS. 1 and 2 with theaddition of the SEA. Preferably, the SEA comprises at least three orfour orthogonally disposed sense electrodes or more than four senseelectrodes disposed around the IPG housing including the IPG connectorblock and the hermetically sealed enclosure. In the depicted example,the SEA comprises sense electrodes 142, 144, and 146 with senseelectrode 146 disposed either on the IPG connector block 112′ or the IPGhermetically sealed housing 118′. As in the embodiment of FIGS. 1 and 2,endocardial leads 114 and 116 transvenously introduced into the rightatrium 12 and right ventricle 16 of the heart 10. The IPG circuit 300′can select the optimal sensing vector sensed by the PVC sense amplifierwithin a sense amplifiers circuit 360′ by an appropriate V-SELECTcommand operating additional PVC sense amplifier input switchingcircuitry of the type disclosed in the above-referenced '966 patent.

The sense electrodes 142, 144, and 146 or the SEA are situated on theIPG housing comprising the connector block 112′ and/or the hermeticallysealed enclosure 118′. Thus, at least four sense vectors arecharacterized as far field sense vectors because the SEA is locatedsubcutaneously remote from the heart 10. The SEA provides three or fourfar field PVC sense vectors comprising PVC sense vector A-B betweensense electrodes 146 and 144, PVC sense vector B-C between senseelectrodes 144 and 142, and PVC sense vector A-C between senseelectrodes 142 and 146 by appropriately coupling the input signals A, B,and C to the PVC sense amplifier inputs within sense amplifiers circuit360′. A fourth PVC sense vector B-(C-A) can be mathematically derivedfrom the input signals A, B and C, but the simpler selection of a pairof input signals among signals A, B, and C may well suffice in practiceand will be assumed in the following description.

The optimal far field sense vector for sensing an R-wave, and, bylogical extension, for sensing PVCs occurring during the PAVBP can bedetermined following implantation of the IPG 100′ and the leads 114 and116 in the patient's body. The sensitivity of the PVC sense amplifierand the V-SELECT pairing signals A, B, and C can both be temporarilyprogrammed in a telemetry session with an external programmer, and theIPG 100′ can be commanded to uplink telemeter the PVC sense signal. Theintrinsic R-waves and any spontaneously occurring PVCs appearing in aconventional ECG display and the PVC sense amplifier's uplinktelemetered response can be observed simultaneously. The PVC senseamplifier sensitivity can be varied for each programmed far field sensevector, and the sense vector providing the best consistent detection ofR-waves with the best ventricular sense safety margin can be determined.Other comparative tests to determine the optimal PVC sense vector couldinclude simply measuring the R wave amplitude, the R wave width, andslew rates through the PVC sense amplifier and determining the optimalPVC sense amplifier through comparison of one or a combination of theseparameters of the sensed R-waves. Or comparative testing can beconducted varying a blanking period applied to the PVC sense amplifierto determine the PVC sense vector across which an R-wave can be sensedat the longest blanking period. A “permanent” V-SELECT can then beprogrammed for coupling the PVC sense amplifier inputs to receive theoptimal pair of signals A, B, C during chronic implantation.

The chronic operation of the selected far field PVC sense vector can bedetermined in a telemetry session initiated at a later time from dataaccumulated in memory registers indicating the number of times that aPVC was detected during the PAVBP and the delivery of a V-PACE wasinhibited at time-out of the PAV delay. In IPGs having the VSP function,the saved data would comprise the number of times that a PVC wasdetected during the PAVBP and/or before time-out of the VSP delay, andthe delivery of a V-PACE pulse at time-out of the VSP delay.

In a similar way, optimal PVC sense vectors can be selected in a dualchamber pacing systems providing right and left heart chamber pacing andsensing of the type described in commonly assigned U.S. Pat. No.6,477,415. Such multi-chamber pacing systems provide right and leftatrial and/or ventricular pacing and sensing particularly to enhancecardiac output of hearts in heart failure. In such a right and leftheart pacing context providing atrial pacing, the PVC sense electrodepair can comprise right and left heart chamber pace/sense electrodesdefining an optimal R-wave sense vector or can additionally comprise oneof the left heart chamber pace/sense electrodes and the indifferentelectrode supported on or comprising the conductive IPG can defining anoptimal PVC sense vector.

Such a right and left heart pacing system comprising endocardial RV lead114, RA lead 116, and a CS lead 150 transvenously introduced into theright ventricle 16, the right atrium 12, and the coronary sinus,respectively of the heart 10 and coupled to the connector block 112″ ofthe IPG 100′″ is depicted in FIGS. 7 and 8. The depicted CS lead 150supports an LV pace/sense electrode 154 disposed in the CS or a coronaryvein descending from the CS in operative relation to the LV and an LApace/sense electrode 152 disposed in the CS in operative relation to theLA.

Pacing and sensing in the RA and RV one or both of the LA and LV can beconducted in the manner described in the above-referenced '415 patent.The components of the IPG circuit 300′″ correspond in large part withthe components of the IPG circuit 300 described above. The flow chartsof FIGS. 3 and 4 are followed, and right and left heart pacing pulsescan be delivered simultaneously or with a delay as determined in block364 of digital controller/timer circuit 330. The output amplifierscircuit 340′ can deliver the depicted RA-PACE, LA-PACE, RA-PACE andRV-PACE pulses through selected pace/sense electrode pairs or employingthe can electrode 140 as an indifferent pacing electrode.

Similarly, the sense amplifiers circuit 360′ includes the respectiveatrial and ventricular sense amplifiers for declaring an LA-EVENT, anRA-EVENT, an LV-EVENT or an RV-EVENT through selected pace/senseelectrode pairs or employing the can electrode 140 as an indifferentsense electrode.

A PVC sense vector can be defined by an appropriate V-SELECT commandthrough pace/sense electrode selection and control circuit or registers350′. In this embodiment illustrated in FIGS. 7 and 8, the PVC sensevector can be selected by an appropriate V-SELECT command among: (1) thecan electrode 140 and the RV ring pace/sense electrode 128; (2) the canelectrode 140 and the LV pace/sense electrode 154; (3) the LA pace/senseelectrode 152 and the LV pace/sense electrode 154; (4) the LA pace/senseelectrode 152 and the RV ring pace/sense electrode 128; and (5) the RVring pace/sense electrode 128 the LV pace/sense electrode 154 depictedas PVC sense vector 160 in FIG. 7. The selection can be made employingcomparative testing of the PVC sense electrode pairs as described above.

In a similar way, PVC sense vectors can be selected in a dual chamberpacing ICD implanted in a patient's chest comprising an ICD IPG andendocardial leads transvenously introduced into the right atrium, rightventricle, and coronary sinus of the heart bearing pace/sense and/orcardioversion/defibrillation electrodes, wherein PVC sensing can beconducted during the PVAB period across selected far field PVC senseelectrode pairs. The dual chamber ICD can also be configured to provideright and left heart pacing and/or have at least one SEA electrodeprovided.

FIGS. 9 and 10 illustrate a dual chamber, multi-programmable, ICD IPG400 and associated lead system for providing atrial and/or ventricularsensing functions for detecting P-waves of atrial depolarizations and/orR-waves of ventricular depolarizations, depending on the programmedpacing and/or sensing mode and delivering pacing orcardioversion/defibrillation therapies. An exemplarycardioversion/defibrillation lead system is depicted in FIG. 9 fordelivering cardioversion/defibrillation shock therapies to the atria orventricles of the heart. FIGS. 9 and 10 are intended to provide acomprehensive illustration of each of the atrial and/or ventricular,pacing and/or cardioversion/defibrillation configurations that may beeffected using sub-combinations of the components depicted therein andequivalents thereto. The present invention can be implemented into suchICDs wherein the R-wave sense amplifier is normally blanked during aPAVBP following delivery of an A-PACE pulse.

In the preferred embodiment of FIGS. 9 and 10, depending on theprogrammed or current pacing mode, pacing pulses are applied to theatrium and/or ventricle in response to the detection of the appropriatebradycardia condition by the ICD IPG operating system. The pacing andsensing functions are effected through atrial and ventricular bipolarpace/sense electrode pairs at the ends of right atrial/superior venacava (RA/SVC) and right ventricular (RV) leads 440 and 416,respectively, fixed in the right atrium 12 and right ventricle 16,respectively, that are electrically coupled to the circuitry of IPG 400through a connector block 412. Delivery of cardioversion ordefibrillation shocks to the atrial and/or ventricular chambers of theheart 10 may be effected through selected combinations of theillustrated exemplary RA and RV cardioversion/defibrillation electrodeson the RA/SVC and RV leads and an additional coronary sinus (CS)electrode on a CS lead 430 as well as an exposed surface electrode 410of the outer housing or can of the IPG 400. The can electrode 410optionally serves as a subcutaneous cardioversion/defibrillationelectrode, used as one electrode optionally in combination with oneintracardiac cardioversion/defibrillation electrode for cardioverting ordefibrillating either the atria or ventricles. A remote, subcutaneousdefibrillation patch electrode may be provided in addition to orsubstitution for the can electrode 410.

The RV lead 416 is depicted in a conventional configuration and includesan elongated insulating lead body, enclosing three concentric,electrically isolated, coiled wire conductors, separated from oneanother by tubular insulating sheaths. Located adjacent the distal endof the RV lead 416 are a pace/sense ring electrode 424, a helical,pace/sense electrode 426, mounted retractably within an insulatingelectrode head 428. Helical electrode 426 is adapted to be extended outof the electrode head 428 and screwed into the ventricular apex in amanner well known in the art. RV pace/sense electrodes 424 and 426 areeach coupled to a coiled wire conductor within the RA lead body and areemployed for cardiac pacing in the ventricle and for sensing near-fieldR-waves. RV lead 416 also supports an elongated, exposed wire coil,cardioversion/defibrillation electrode 422 in a distal segment thereofadapted to be placed in the right ventricle 16 of heart 10. The RVcardioversion/defibrillation electrode 422 may be fabricated fromplatinum, platinum alloy or other materials known to be usable inimplantable cardioversion/defibrillation electrodes and may be about 5cm in length. The cardioversion/defibrillation electrode 422 is alsocoupled to one of the coiled wire conductors within the lead body of RVlead 416. At the proximal end of the lead body is a bifurcated connectorend 418 having three exposed electrical connectors, each coupled to oneof the coiled conductors that are attached within the connector block412 to connector block terminals in a manner well known in the art.

The coronary sinus (CS) lead 430 includes an elongated insulating leadbody enclosing one elongated coiled wire conductor coupled to anelongated exposed coil wire cardioversion/defibrillation electrode 434.CS cardioversion/defibrillation electrode 434, illustrated in brokenoutline, is located within the coronary sinus and great vein 408 of theheart 10 and may be about 5 cm in length. At the proximal end of the CSlead 430 is a connector end 432 having an exposed connector coupled tothe coiled wire conductor and attached within the connector block 412 toconnector block terminals in a manner well known in the art.

The RA/SVC lead 440 includes an elongated insulating lead body carryingthree concentric, electrically isolated, coiled wire conductorsseparated from one another by tubular insulating sheaths, correspondinggenerally to the structure of the RV lead 416. The lead body is formedin a manner well known in the art in an atrial J-shape in order toposition its distal end in the right atrial appendage. A pace/sense ringelectrode 444 and an extendable helical, pace/sense electrode 446,mounted retractably within an insulating electrode head 448, are formeddistally to the bend of the J-shape. Helical electrode 446 is adapted tobe extended out of the electrode head 448 and screwed into the atrialappendage in a manner well known in the art. RA pace/sense electrodes444 and 446 are employed for atrial pacing and for near-field sensing ofP-waves. An elongated, exposed coil defibrillation RA/SVC electrode 450is supported on RA lead 440 extending proximally to pace/sense ringelectrode 444 and coupled to the third coiled wire conductor within theRA lead body. Electrode 450 preferably is 40 cm in length or greater andis configured to extend from within the SVC and toward the tricuspidvalve. At the proximal end of the RA lead 440 is a bifurcated connector442 which carries three exposed electrical connectors, each coupled toone of the coiled wire conductors, that are attached within theconnector block 412 to connector block terminals in a manner well knownin the art.

Preferably, bipolar pace/sense electrodes 444, 446 and 424, 426 areemployed for near field sensing and for delivery of pacing pulses to theatria and ventricles. The configuration, manner of fixation, andpositioning of bipolar pace/sense electrodes 444, 446 and 424, 426 withrespect to the atria and ventricles, respectively, may differ from thoseshown in FIG. 9. Unipolar pace/sense electrode bearing leads may also beused in the practice of the invention, and the second, return electrodemay be one or more of the cardioversion/defibrillation electrodes or thecan electrode 410.

The ICD system configuration and operating modes of FIG. 9 may be variedby eliminating: (1) the atrial or ventricularcardioversion/defibrillation capability and associated lead andelectrodes while retaining the dual chamber pacing and sensingcapability thereby providing single chamber cardioversion/defibrillationand dual chamber bradycardia/tachycardia pacing capabilities; or (2) ina special case of an atrial ICD, the ventricularcardioversion/defibrillation capability while retaining at least theatrial pace/sense capability and the ventricular sense capability forproviding R-wave synchronization of the delivered atrial cardioversiontherapies. In each such system, it will be understood that appropriatedefibrillation and pacing leads will be employed in the system. In asimpler ICD system employing only the IPG can electrode 410 or acardioversion/defibrillation electrode implanted subcutaneously and moreremote from the heart chamber and only one the other of thecardioversion/defibrillation electrode located in proximity to theatrium or ventricle, e.g. electrodes 422 or 450, then it is desirable tocouple the PVC sense amplifier inputs to the availablecardioversion/defibrillation electrodes.

FIG. 10 is a functional schematic diagram of the circuitry of a dualchamber, ICD 400 in which the present invention may usefully bepracticed. The circuitry of FIG. 10 should be taken as exemplary of adual chamber ICD IPG 400 in which the invention may be embodied, and notas limiting, as it is believed that the invention may usefully bepracticed in a wide variety of device implementations, as long as a dualchamber pacing mode providing bradycardia pacing therapies to the atriais retained that involve blanking of the ventricular sense amplifier.

The ICD IPG circuitry of FIG. 10 includes a high voltage section forproviding relatively high voltage cardioversion/defibrillation shockswhen needed in response to detection of a tachyarrhythmia, a low voltagepace/sense section for sensing P-waves and/or R-waves and providingrelatively low voltage bradycardia pacing and anti-tachycardia pacingtherapies, both operated under the control of a microcomputer includinga microprocessor 224, ROM/RAM 226 and DMA 228. Other functions,including uplink and downlink telemetry with an external programmer forinterrogating or programming operating modes and parameters, are alsoprovided (but not shown) in a manner well known in the art.

The block diagram of FIG. 10 depicts the atrial and ventricularpace/sense and defibrillation lead connector terminals of the connectorblock 412. Assuming the electrode configuration of FIG. 9, thecorrespondence to the illustrated leads and electrodes is as follows:Optional terminal 310 is hard wired to can electrode 410, that is, theun-insulated portion of the housing of the ICD IPG 400, and technicallymay be directly connected and not be part of the connector block 412.Terminal 320 is adapted to be coupled through RV lead 416 to RVcardioversion/cardioversion/defibrillation electrode 422. Terminal 311is adapted to be coupled through RA lead 440 to RA/SVC electrode 450.Terminal 318 is adapted to be coupled through CS lead 430 to CScardioversion/defibrillation electrode 434. However, it will beunderstood that fewer terminals may be provided than depicted, and/orthat one or more differing defibrillation leads, e.g. epicardial patchelectrode and subcutaneous patch electrode bearing leads may also beemployed for one or more of the depicted cardioversion/defibrillationelectrode bearing leads.

Terminals 310, 311, 318 and 320 are coupled to high voltage outputcircuit 234. High voltage output circuit 234 includes high voltageswitches controlled by CV/DEFIB CONTROL logic 230 via control bus 238.The switches within circuit 234 control which electrodes are employedand which are coupled to the positive and negative terminals of thecapacitor bank including capacitors 246 and 248 during delivery of theintermediate and high voltage cardioversion and defibrillation shocks.

Terminals 324 and 326 of the connector block are adapted to be coupledthrough RV lead 416 to RV pace/sense electrodes 424 and 426 for sensingand pacing in the ventricle. Terminals 317 and 321 are adapted to becoupled through RA/SVC lead 440 to RA pace/sense electrodes 444 and 446for sensing and pacing in the atrium. Terminals 324 and 326 are coupledto the inputs of R-wave sense amplifier 200 through switches in switchnetwork 208. R-wave sense amplifier 200, which preferably takes the formof an automatic gain controlled amplifier providing an adjustablesensing threshold as a function of the measured R-wave signal amplitude.A VSENSE signal is generated on R-OUT line 202 whenever the signalsensed between electrodes 424 and 426 exceeds the current ventricularsensing threshold.

Terminals 317 and 321 are coupled to the P-wave sense amplifier 204through switches in switch network 208. P-wave sense amplifier 204preferably also takes the form of an automatic gain controlled amplifierproviding an adjustable sensing threshold as a function of the measuredP-wave amplitude. An ASENSE signal is generated on P-OUT line 206whenever the signal sensed between pace/sense electrodes coupled toterminals 317, 321 exceeds the current atrial sensing threshold.

The A-PACE and V-PACE output circuits 214 and 216 are also coupled toterminals 317, 321 and 324, 326, respectively. The atrial andventricular sense amplifiers 204 and 200 are isolated from the A-PACEand V-PACE output circuits 214 and 216 by appropriate isolation switcheswithin switch matrix 208 and also by blanking circuitry operated byA-BLANK and V-BLANK signals during and for a short time followingdelivery of a pacing pulse in a manner well known in the art. One of theV-BLANK signals is the post atrial ventricular blanking signal providedduring the PAVBP period as described above with reference to the dualchamber pacemaker IPG 100. The general operation of the R-wave andP-wave sense amplifiers 200 and 204 may correspond to that disclosed inU.S. Pat. No. 5,117,824, for example.

The ICD IPG circuitry of FIG. 10 provides atrial and/or ventricularcardiac pacing for bradycardia and tachycardia conditions andsynchronized cardioversion and defibrillation shock therapies fortachyarrhythmias in accordance with therapy regimes programmed by thephysician. With respect to the pacing operations, the pacer timing andcontrol circuitry 212 includes programmable digital counters whichcontrol the basic time intervals associated with bradycardia pacingmodes as is well known to the art.

In normal pacing modes of operation, e.g., the dual chamber pacing modeas set forth in FIG. 3, for example, intervals defined by pacer timingand control circuitry 212 include atrial and ventricular pacing escapeintervals, blanking intervals, including the PAVBP, the refractoryperiods during which sensed P-waves and R-waves are ineffective torestart timing of the escape intervals, and the pulse widths of thepacing pulses. These intervals are determined by microprocessor 224, inresponse to stored data in RAM in ROM/RAM 226 and are communicated tothe pacer timing and control circuitry 212 via address/data bus 218.Pacer timing and control circuitry 212 also determines the amplitude ofthe cardiac pacing pulses under control of microprocessor 224.

During pacing, the escape interval counters within pacer timing andcontrol circuitry 212 are reset upon sensing of R-waves and P-waves asindicated by a signals on lines 202 and 206. In accordance with theselected pacing mode, pacer timing and control circuitry 212 providespace trigger signals to the A-PACE and V-PACE output circuits 214 and216 on timeout of the appropriate escape interval counters to triggergeneration of atrial and/or ventricular pacing pulses. The pacing escapeinterval counters are also reset on generation of A-PACE and R-PACEpulses, and thereby control the basic timing of cardiac pacingfunctions.

Pacer timing and control circuitry 212 also controls escape intervalsassociated with timing and delivering anti-tachyarrhythmia pacing inboth the atrium and the ventricle, employing any anti-tachyarrhythmiapacing therapies known to the art. The value of the counts present inthe escape interval counters when reset by sensed R-waves and P-wavesmay be used as measures of the durations of R-R intervals, P-Pintervals, P-R intervals and R-P intervals, which measurements arestored in RAM in ROM/RAM 226 and used to detect the presence oftachyarrhythmias as described below.

Microprocessor 224 operates as an interrupt driven device, and isresponsive to interrupts from pacer timing and control circuitry 212corresponding to the occurrence sensed P-waves (ASENSE) and R-waves(VSENSE) and corresponding to the generation of cardiac pacing pulses.These interrupts are provided via data/address bus 218. Any necessarymathematical calculations to be performed by microprocessor 224 and anyupdating of the values or intervals controlled by pacer timing/controlcircuitry 212 take place following such interrupts.

For example, in response to a sensed or paced ventricular depolarizationor R-wave, the intervals separating that R-wave from the immediatelypreceding R-wave, paced or sensed (R-R interval) and the intervalseparating the paced or sensed R-wave from the preceding atrialdepolarization, paced or sensed (P-R interval) may be stored. Similarly,in response to the occurrence of a sensed or paced atrial depolarization(P-wave), the intervals separating the sensed P-wave from theimmediately preceding paced of sensed atrial contraction (P-P Interval)and the interval separating the sensed P-wave from the immediatelypreceding sensed or paced ventricular depolarization (R-P interval) maybe stored. Preferably, a portion of RAM in the ROM/RAM 226 (FIG. 10) isconfigured as a plurality of recirculating buffers, capable of holding apreceding series of measured intervals, which may be analyzed inresponse to the occurrence of a pace or sense interrupt to determinewhether the patient's heart is presently exhibiting atrial orventricular tachyarrhythmia.

Detection of atrial or ventricular tachyarrhythmias, as employed in thepresent invention, may correspond to tachyarrhythmia detectionalgorithms known to the art. For example, presence of atrial orventricular tachyarrhythmia may be confirmed by means of detection of asustained series of short R-R or P-P intervals of an average rateindicative of tachyarrhythmia or an unbroken series of short R-R or P-Pintervals. The suddenness of onset of the detected high rates, thestability of the high rates, or a number of other factors known to theart may also be measured at this time.

In the event that an atrial or ventricular tachyarrhythmia is detected,and an anti-tachyarrhythmia pacing regimen is prescribed, appropriatetiming intervals for controlling generation of anti-tachyarrhythmiapacing therapies are loaded from microprocessor 224 into the pacertiming and control circuitry 212, to control the operation of the escapeinterval counters therein and to define refractory periods during whichdetection of R-waves and P-waves is ineffective to restart the escapeinterval counters.

In the event that generation of a cardioversion or defibrillation shockis required, microprocessor 224 employs the an escape interval counterto control timing of such cardioversion and defibrillation pulses, aswell as associated refractory periods. In response to the detection ofatrial or ventricular fibrillation or tachyarrhythmia requiring acardioversion pulse, microprocessor 224 activatescardioversion/defibrillation control circuitry 230, which initiatescharging of the high voltage capacitors 246 and 248 via charging circuit236, under control of high voltage charging control line 240. Thevoltage on the high voltage capacitors is monitored via VCAP line 244,and the monitored voltage signal is passed through multiplexer 220,digitized, and compared to a predetermined value set by microprocessor224 in ADC/comparator 222. When the voltage comparison is satisfied, alogic signal on Cap Full (CF) line 254 is applied tocardioversion/defibrillation control circuit 230, terminating charging.Thereafter, timing of the delivery of the defibrillation orcardioversion shock is controlled by pacer timing/control circuitry 212.Following delivery of the fibrillation or tachycardia therapy, themicroprocessor 224 then returns the operating mode to cardiac pacing andawaits the next successive interrupt due to pacing or the occurrence ofa sensed atrial or ventricular depolarization.

In the illustrated ICD operating system, delivery of the cardioversionor defibrillation shocks is accomplished by output circuit 234, undercontrol of control circuitry 230 via control bus 238. Output circuit 234determines whether a monophasic or biphasic shock is delivered, thepolarity of the electrodes and which electrodes are involved in deliveryof the shock. Output circuit 234 also includes high voltage switchesthat control whether electrodes are coupled together during delivery ofthe shock. Alternatively, electrodes intended to be coupled togetherduring the shock may simply be permanently coupled to one another,either exterior to or interior of the device housing, and polarity maysimilarly be pre-set, as in current implantable defibrillators. Anexample of output circuitry for delivery of biphasic shock regimens tomultiple electrode systems may be found in U.S. Pat. No. 4,727,877, forexample.

In accordance with the present invention, a PVC sense amplifier 210 isincorporated into the circuitry of FIG. 10 having a pair of sense inputsthat can be selectively coupled through switches within switch network208 in response to a programmed V-SELECT command received through bus218 to a pair of PVC sense electrodes selected from among the depictedelectrodes, preferably from among cardioversion/defibrillationelectrodes 422, 434, 450, can electrode 410, and one of the ringpace-sense electrode 424 and the tip pace/sense electrode 426. Switchmatrix 208 is used in the PVC sensing function of the present inventionto select which pair of the available pace/sense and/orcardioversion/defibrillation electrodes is coupled to the inputs of wideband (0.5-200 Hz) PVC sense amplifier 210 for use in detecting PVCsduring the PAVBP (and at other times during the cardiac cycle). A PVCsignal from bandpass amplifier 210 is passed through multiplexer 220 andmay be converted to multi-bit digital signals by ND converter 222, forstorage in RAM in ROM/RAM 226 under control of DMA 228. Themicroprocessor 224 may employ digital signal and morphology analysistechniques to characterize the digitized signals stored in ROM/RAM 226to recognize and classify the patient's heart rhythm employing any ofthe numerous signal processing methodologies known to the art.

In a dual chamber pacing mode involving atrial pacing and ventricularsensing, the PVC amplifier 210 is not blanked during the PAVBP. Thesteps set forth in FIGS. 3 and 4 are followed. A PVC that is detectedduring time-out of a PAV is employed as an interrupt to themicroprocessor 224 in step S130. The steps of FIG. 4 are followed todetermine whether to inhibit the delivery of a V-PACE pulse upontime-out of the PAV delay or to deliver a V-PACE upon time out of a VSPdelay.

In this embodiment illustrated in FIGS. 9 and 10, the PVC sense vectorcan be selected by an appropriate V-SELECT command among: (1) the canelectrode 410 and the RV coil cardioversion/defibrillation electrode422; (2) the can electrode 410 and the SVC coilcardioversion/defibrillation electrode 450; (3) the can electrode 410and the RV ring pace/sense electrode 424; (4) the can electrode 410 andthe CS coil cardioversion/defibrillation electrode 434; (5) the CS coilcardioversion/defibrillation electrode 434 and the RV ring pace/senseelectrode 424; (6) the CS coil cardioversion/defibrillation electrode434 and the RV coil cardioversion/defibrillation electrode 422; and (7)the RV coil cardioversion/defibrillation electrode 422 and the SVC coilcardioversion/defibrillation electrode 450.

Advantageously, the PVC sense amplifiers within the sense amplifierscircuits 360, 360′, and 360″ and the PVC sense amplifier 210 and can beenabled during the cardiac cycle to function as a conventional EGM senseamplifier so that the spontaneously occurring PQRST complexes can berecorded for real time analysis or data storage as is well known in theart.

All patents and publications referenced herein are hereby incorporatedby reference in their entireties.

It will be understood that certain of the above-described structures,functions and operations of the above-described preferred embodimentsare not necessary to practice the present invention and are included inthe description simply for completeness of an exemplary embodiment orembodiments.

In addition, it will be understood that specifically describedstructures, functions and operations set forth in the above-referencedpatents can be practiced in conjunction with the present invention, butthey are not essential to its practice. It is therefore to beunderstood, that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described withoutactually departing from the spirit and scope of the present invention.

1. A pacing system comprising: an implantable pulse generator, an atriallead coupled to the implantable pulse generator having at least oneactive atrial pace/sense electrode adapted to be disposed in operativerelation to an atrial heart chamber, at least one indifferent atrialpace/sense electrode adapted to be implanted in the patient's body, aventricular lead coupled to the implantable pulse generator having atleast one active ventricular pace/sense electrode adapted to be disposedin operative relation to a ventricular heart chamber, and an indifferentventricular pace/sense electrode adapted to be implanted in thepatient's body; means for generating atrial pace (A-PACE) pulses; meansfor sensing a natural ventricular depolarization; means for producing aV-EVENT signal upon sensing a natural ventricular depolarization; meansfor generating ventricular pace (V-PACE) pulses; V-A interval timingmeans for timing out a V-A interval following generation of a V-PACEpulse and following sensing of a V-EVENT; means for triggering saidatrial pace generating means to generate an A-PACE pulse followingexpiration of the V-A interval; means for timing out a PAV interval upontriggering of said atrial pace generating means; means for timing aventricular blanking period following generation of an atrial pace pulse(A-PACE); means for inhibiting said V-EVENT signal producing meansduring the ventricular blanking period following generation of an atrialpace pulse (A-PACE); a PVC sense electrode pair spatially separated fromthe ventricular pace/sense electrodes; sensing means coupled to the PVCsense electrode pair for generating a signal indicating occurrence of aPVC, as a PVC declaration signal, by sensing of a depolarization of theventricles occurring during the ventricular blanking period; and meansresponsive to the generation of a PVC declaration signal for terminatingthe PAV delay and triggering the V-A interval timing means to time outthe V-A interval.
 2. The pacing system of claim 1, further comprising:means for timing a ventricular safety pace delay from the delivery of anA-PACE pulse; and means responsive to a declaration of a PVC fortriggering the ventricular pace means to generate and deliver a V-PACEpulse through the active and indifferent ventricular pace/senseelectrodes upon time-out of the ventricular safety pace delay.
 3. Thepacing system of claim 2, wherein the ventricular safety pace delay islonger than the ventricular blanking period and is shorter than the PAVdelay and is selected to ensure that the delivered V-PACE pulse is notdelivered into the vulnerable period of the heart.
 4. The pacing systemof claim 1, further comprising: atrial sensing means coupled to theactive and indifferent atrial pace/sense electrodes for sensing naturalatrial depolarizations and declaring an A-EVENT; means for terminatingthe V-A interval upon declaration of an A-EVENT during time-out of theV-A interval; and means for timing out an SAV interval upon declarationof an A-EVENT during time-out of the V-A interval.
 5. The pacing systemof claim 1, further comprising: means for timing a ventricular safetypace delay from the delivery of an A-PACE pulse, the ventricular safetypace delay is longer than the ventricular blanking period and is shorterthan the PAV delay and is selected to ensure that the delivered V-PACEpulse is not delivered into the vulnerable period of the heart; andmeans responsive to a declaration of a PVC during the ventricular safetypace delay for triggering the ventricular pace pulse generating means togenerate and deliver a V-PACE pulse through the active and indifferentventricular pace/sense electrodes to the ventricular heart chamber upontime-out of the ventricular safety pace delay.
 6. The pacing system ofclaim 1, wherein: the active and indifferent ventricular pace/senseelectrodes are disposed on the ventricular lead; and the PVC senseelectrode pair comprises an indifferent pace/sense electrode disposed onthe implantable pulse generator and one of the active and indifferentventricular pace/sense electrodes are disposed on the ventricular lead.7. The pacing system of claim 1, wherein: the implantable pulsegenerator comprises a housing supporting at least two sense electrodesin a sense electrode array; and the PVC sense electrode pair comprisesthe sense electrodes supported by the implantable pulse generatorhousing.
 8. The pacing system of claim 1, wherein the implantable pulsegenerator comprises a housing supporting at least three sense electrodesin a sense electrode array, and further comprising: means of selectingthe PVC sense electrode pair from among the at least three senseelectrodes supported by the implantable pulse generator housing.
 9. Thepacing system of claim 1, further comprising a first and secondcardioversion/defibrillation electrodes disposed about one of the atriaand the ventricles for delivering cardioversion/defibrillation shocks tothe heart chamber, and wherein: the implantable pulse generator furthercomprises means for determining the existence of a tachyarrhythmia andproviding cardioversion/defibrillation shock therapy through the firstand second cardioversion/defibrillation electrodes; and the PVC senseelectrodes include at least one of the first and secondcardioversion/defibrillation electrodes.
 10. The pacing system of claim9, wherein the PVC sense electrodes include the first and secondcardioversion/defibrillation electrodes.
 11. The pacing system of claim1, wherein: the atrial lead supports a firstcardioversion/defibrillation electrode adapted to be disposed inrelation to the heart, and the ventricular lead supports a secondcardioversion/defibrillation electrode adapted to be disposed inrelation to the heart; the implantable pulse generator further comprisesmeans for determining the existence of a tachyarrhythmia and providingcardioversion/defibrillation shock therapy through the first and secondcardioversion/defibrillation electrodes; and the PVC sense electrodesinclude at least one of the first and secondcardioversion/defibrillation electrodes.
 12. The pacing system of claim11, wherein the PVC sense electrodes include the first and secondcardioversion/defibrillation electrodes.
 13. The pacing system of claim1, further comprising a coronary sinus lead extending from theimplantable pulse generator supporting a firstcardioversion/defibrillation electrode adapted to be disposed inrelation to a left heart chamber of the heart, and wherein: theventricular lead supports a second cardioversion/defibrillationelectrode adapted to be disposed in relation to the right ventricle ofthe heart; the implantable pulse generator further comprises means fordetermining the existence of a tachyarrhythmia and providingcardioversion/defibrillation shock therapy through the first and secondcardioversion/defibrillation electrodes; and the PVC sense electrodesinclude at least one of the first and secondcardioversion/defibrillation electrodes.
 14. The pacing system of claim13, wherein the PVC sense electrodes include the first and secondcardioversion/defibrillation electrodes.
 15. The pacing system of claim1, further comprising a coronary sinus lead extending from theimplantable pulse generator supporting a left heart chamber pace/senseelectrode adapted to be disposed in relation to a left heart chamber ofthe heart, and wherein: the implantable pulse generator furthercomprises means for providing synchronized pacing of right and leftheart chambers through the atrial and ventricular pace/sense electrodesand the left heart chamber pace/sense electrode; and the PVC senseelectrodes include the left heart chamber pace/sense electrode.
 16. Thepacing system of claim 15, wherein the PVC sense electrodes include theleft heart chamber pace/sense electrode and one of the active andindifferent ventricular pace/sense electrodes.
 17. The pacing system ofclaim 15, wherein the PVC sense electrodes include the left heartchamber pace/sense electrode and an indifferent pace/sense electrodedisposed on the implantable pulse generator.
 18. A pacing systemcomprising: an atrial electrode, a ventricular electrode and a PVCsensing electrode, spatially separated from one another and all coupledto an implantable pulse generator, the implantable pulse generatorcomprising: an atrial pulse generator coupled to the atrial electrode; aventricular blanking timer initiated responsive to the atrial pulsegenerator, initiated responsive to generation of an atrial pacing pulse;and a V-A interval timer responsive to and initiated by signals sensedon the ventricular electrode after but not during timeout of theventricular blanking timer and triggering generation of an atrial pacingpulse on expiration of the V-A interval; wherein the V-A interval timeris further responsive to and initiated by signals sensed on the PVCsense electrode during timeout of the ventricular blanking timer. 19.The system of claim 18 wherein the PVC sense electrode comprises anelectrode located on a transvenous lead.
 20. The system of claim 18wherein the PVC sense electrode comprises an electrode located on asubcutaneous lead.
 21. The system of claim 18 wherein the ventricularelectrode comprises an electrode located on a left ventricular lead. 22.The system of claim 18 wherein the ventricular electrode comprises anelectrode located on a right ventricular lead.
 23. The system of claim18 wherein: the V-A interval timer is further responsive to andinitiated by signals sensed on the PVC sense electrode after the timeoutof the ventricular blanking timer.
 24. The system of claim 18 whereinthe ventricular blanking interval is 400 milliseconds or less.
 25. Thesystem of claim 18 wherein the ventricular blanking interval is about 30milliseconds.
 26. The system of claim 18 further comprising: aventricular pulse generator coupled to the ventricular electrode; aventricular safety pacing interval timer responsive to and triggered bythe atrial pulse generator; and a ventricular safety pacing flag set inresponse to signals sensed on the PVC sense electrode during ventricularsafety pacing interval timeout; and wherein the ventricular pulsegenerator is responsive to and triggered by expiration of theventricular safety pacing interval if the ventricular safety pacing flagwas set during the ventricular safety pacing interval.
 27. The system ofclaim 26 further comprising: an A-V pacing interval timer responsive tothe atrial pulse generator and initiated by generation of an atrialpacing pulse; wherein the ventricular pulse generator is responsive toand triggered by the expiration of the A-V pacing interval; and whereinthe pace A-V interval interval timer is responsive to and timing of theA-V interval is terminated by signals sensed on the PVC sense electrodeafter expiration of the ventricular safety pacing interval.